Armed only with a bucket full of enthusiasm, a wiki full of miss-information and a 2 digit IQ, the good Captain seeks not only to understand the modern world, but also fix one or two bits of kit along the way.

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Monday, 9 January 2017

Garmin eTrex 10: what's your position buddy?

It's struck me once or twice, when out walking, that I don't know exactly where I am.

Imagine that there is some kind of medical emergency, and I need to call for an ambulance.

"Where are you sir?"
"Oh, I'm somewhere in the New Forest!"

How on earth would they find me?

Bring on the gps

As mentioned in earlier posts, we take our Garmin with us when out walking in the English countryside, even if only to record a waypoint so we can find our way back to the car. And since I have discovered how to install free topographical maps on our basic eTrex 10, I have taken a lot more interest it how it works and what it has to offer.

I've noticed a lot of people on forums saying that the user manual for the eTrex is rubbish. However, the manual is not badly written. The problem is that its written on the assumption that the reader knows what the device is capable of, and knows what he or she wants to do.

The user manual is not written for people like me. People that bought the device with only a vague notion of what they might be able to use it for, and a genetic resistance to reading user manuals. What we need is a "How would I do that?" guide. If I can find a couple of days with nothing more interesting to do, I might write one.

In the meantime I'll just struggle on, picking up bits of information here and there.

Location, location, location

To read our current location, we need to start by setting the format.

Navigate from top level menu to Setup > Position Format and set Position Format to British Grid. The Map Datum should now be listed as Ord Srvy GB. Also, Map Spheroid will be Airy (don't bother to ask, I've no idea!).

Now hit the Back button a few times to get back to the top level menu. There are now a few ways to display your current location:-

1. From any of the screens you should now be able to read the coordinates for your current position by just pressing and holding the joystick button for a few seconds. The display should change and include your Location like this:-

TQ 20554BNG 33968

Note: If you press the joystick again, it will select the default option ("Done") and save a waypoint. Alternatively, just press the "back" button.

2. The Satellite screen also shows the current location.

3. Your "Map" screen may have already been setup to include the current location coordinates.

Caution: sometimes (when using the joystick on the Map) coordinates appear at the top of the map. These are the coordinates of the cursor on the map, not necessarily your actual current coordinates.

What do these funny numbers mean?

Briefly, BNG = British National Grid, and in my example, TQ just happens to represent a 100km square of Great Britain in the south east of England.

The pair of numbers represent the distances from the south west corner of the 100km square TQ. The first 5 digits are the easterly distance from the south west corner of TQ, and the second set of 5 digits represents the northerly distance from the SW corner of TQ.

Because there are 5 digits in each number in the pair, the resolution is 1m (i.e. it would be 10km for 1 digit, 1km for 2 digits, 100m for 3, and 10m for 4 digits).

Since the resolution is 1m, the coordinates define the SW corner of a 1m square. But remember that the best gps resolution is 3m, so these coordinates are (as near as damn it) your location.

For a more detailed (and probably more accurate) explanation, see this Wikipedia article.

You can take BNG coordinates and type them into a website like GridReferenceFinder to check the location on a map or satellite view of the Earth. For example;

if you take the first 3 digits for the data pair above (i.e. TQ 205 339) and type them into GridReferenceFinder you should see a point on the map which is within 100m of the Frog and Nightgown pub

if you take the first 4 digits (i.e. TQ 2055 3397) the point on the map should be within 10m of the pub

Probably more useful, you can take the coordinates for an "off the beaten track" pub from the internet and program them into your eTrex;

from the top level menu select Mark Waypoint > Coordinates

select Location using joystick

edit the existing reference by using the joystick to navigate and select the required coordinates

the left/right arrow functions are used to select a reference letter or digit position within the string

the up/down arrow functions are used to select the alpha code (in my case "TQ")

and the numbers are (well, naturally) for the coordinate digits

you may only have a 6 digit reference, so pad it with zeroes (e.g. for TQ 206 372 enter TQ 20600 37200)

You can also edit the waypoint number to create a user friendly name (e.g. The Star Inn) and change the icon for something resembling a pub.

click Done when you have finished

So you now have a pub waypoint on your Map...

...time for a drink!

When you get to the pub you can update the waypoint to make it more accurate, based upon your actual location (e.g. bar, carpark, gents).

Display what you like

It is easy to customise the Map view on the eTrex 10. So if your current coordinates are really important to you, they can be selected to show above the map.

Just select Setup > Map > Data Fields and press the joystick

Select 2 small and then "back" a couple of times to get back to the top level menu

Now select Map > Menu > Change Data Fields

Select the data panel you would like to change and then press the joystick

Use the joystick to select the parameter to display

There seem to be 38 different parameters to choose from including Location (selected) which is the location in the format selected.